Certain industrial processes depend on maintaining environments free of atmospheric contamination. One example is in the field of semiconductor device manufacturing, which uses a wide variety of environments for processing wafers into integrated circuits (ICs). Many of these processing environments can be significantly degraded with even very small amounts of oxygen, moisture, and/or nitrogen, all of which are present in air. Consequently, even small leaks that allow air into the processing chamber can be very deleterious to the processing of these wafers.
Chemical vapor deposition (CVD) of silicon and silicon nitride layers onto the silicon wafer are particularly important semiconductor processes that depend on maintaining a process environment as free of atmospheric leaks as possible. Typically, these layers are deposited inside of a vacuum chamber, using gases such as silane or dichlorosilane, which react inside the process chamber to form the deposited layers on the silicon wafers.
If the process environment is very pure, high quality layers can be deposited; however, if even small amounts of oxygen or moisture are introduced into the chamber, the reaction will also form silicon oxide. This silicon oxide can be directly incorporated into the deposited layer, where it will alter the properties of the deposited layer, or it can take the form of particles that can fall onto the silicon wafer and ultimately disrupt the operation of the ICs.
There are many potential causes of leaks in these processing chambers, including damaged or poorly seated O-rings, leaking valves, leaking gas delivery systems, etc. Since there are many sources, the occurrence of a leak is extremely difficult to predict; consequently, it is highly desirable to be able to detect the presence of a leak prior to carrying out the specific process step, such as the chemical vapor deposition of a silicon or silicon nitride layer.
One very straightforward approach to detecting the presence of an atmospheric leak is to use a measurement technique that can detect very small amounts of oxygen or moisture inside the process chamber. Typical techniques that one could use include optical emission spectroscopy, infrared absorption, or residual gas analysis. One could also use these techniques to measure nitrogen; however, many processing environments already contain nitrogen as one of the gases used in the process, particularly if the deposited layer contains nitrogen, such as in the case of a silicon nitride layer.
Another approach for detecting the presence of a leak is to pump down the chamber to a low pressure, then completely seal off the chamber from the pump, and measure the rate of rise of pressure. In this case, it is assumed that any pressure rise is attributable to air entering the chamber.
Unfortunately, these measurements can be inaccurate if there is any oxygen or moisture entering the process chamber environment in addition to the atmospheric leak. Particularly in the silicon dioxide or alumina process chambers used for the deposition of silicon and silicon nitride layers, gases such as oxygen and moisture can be adsorbed onto the chamber walls whenever the chamber walls are exposed to these gases. Exposure to these gases can occur during loading and unloading of the wafers. In subsequent steps, when the chamber is pumped down to low processing pressures, any oxygen or moisture that is present on the walls will desorb over time, thus entering the process environment. This process is referred to as “outgassing”.
Although outgassing of oxygen or moisture is undesirable, it is not nearly as deleterious as a leak. In the case that oxygen or moisture is adsorbed on the chamber wall, during the first few seconds of the silicon or silicon nitride deposition process the silane or dichlorosilane will quickly react with these gases and form a layer that covers these adsorbed gases. In the case of a leak, however, the oxygen and moisture will continue to be present during the entire deposition process.
Since atmospheric leaks are much more deleterious than outgassing, it would be desirable to be able to differentiate between the two sources of oxygen and moisture contamination. Current techniques, however, cannot differentiate between the two sources of contamination. From the above, it is seen that improved techniques for measuring atmospheric leaks in process chambers are desired.